In order to enhance charge mobility of a silicon semiconductor, existing semiconductor companies and various research groups have attempted various methods, and as a representative method, silicon is grown on a silicon-germanium alloy with a crystal lattice constant. If this method is used, a significant amount of tensile stress is applied to a silicon thin film due to lattice distortion of silicon-germanium and silicon, and resultant improvement of charge mobility may be expected. However, a silicon-germanium, also silicon thereon, should be grown again difficulty, and complex processes and great costs are also problems.
In case of an existing silicon-based flexible field effect transistor (FET) prepared on a flexible substrate such as an existing plastic or rubber substrate, a silicon oxide (SiO2) sacrificial layer is etched from a silicon thin film wafer formed on an insulator in a silicon-on-insulator (SOI) wafer in order to separate the silicon thin film, and is transferred onto a flexible substrate. In this method, since silicon oxide at a lower portion is used as a sacrificial layer for etching, compressive stress present in the silicon oxide is removed while silicon is being separated. Also, since silicon at an upper portion is suspended in a suspended state, no stress is applied. Therefore, improvement of charge mobility caused by a strain rate is not expected.